Rubber compositions are often reinforced with reinforcing fillers such as at least one of rubber reinforcing carbon black and synthetic amorphous silica (e.g. precipitated silica).
Various products contain at least one component comprised of such rubber compositions such as, for example, tires.
In order to enhance rubber reinforcing effects of precipitated silica, a coupling agent is typically used in combination with the precipitated silica.
Such coupling agent typically contains a moiety (e.g. alkoxysilane group) reactive with hydroxyl groups (e.g. silanol groups) on the precipitated silica and another different moiety (e.g. polysulfide as a sulfur contributing moiety) interactive with elastomers containing carbon-to-carbon double bonds (e.g. diene-based elastomers).
A typical disadvantage of such polysulfide moiety of the silica coupling agent is its sulfur contribution at an elevated temperature of the uncured rubber composition, such as for example during physical mixing of the uncured rubber composition, which interacts with carbon-to-carbon double bonds of an elastomer in the rubber composition to promote a significantly increased viscosity of the rubber composition which leads to increased rubber processing difficulties, or challenges. Such phenomenon is well known to those having skill in such art.
For this invention, a rubber reinforcing precipitated silica is provided as a precipitated silica treated with a combination of allylsilane and dialkylsilane. It appears that such combination can produce a synergistic effect for the treated silica insofar as a tan delta physical property of a rubber composition is concerned.
A dialkylsilane for such treatment may be, for example, in a form of a dialkyldihalosilane such as for example a dimethyldichlorosilane or a dimethyldialkoxysilane such as, for example, a dimethyldiethoxysilane. An allylsilane for such treatment may, for example, be in a form of an allyltrialkoxysilane or allylhalosilane.
For this invention, it has been discovered that a precipitated silica treated with a combination of allylsilane and dialkylsilane may be used as silica reinforcement for a sulfur curable rubber composition without use of a sulfur-containing silica coupling agent. Such treated precipitated silica does not contain a polysulfidic moiety so that sulfur is not available to prematurely interact with the elastomer(s) in the rubber composition.
While the mechanism might not be fully understood, where precipitated silica is treated with an allylsilane such as for example, an allyltrialkoxysilane or allylhalosilane, together with a dialkyldihalosilane such as, for example, dimethyldichlorosilane, it is envisioned that an alkoxysilane moiety of the allylalkoxysilane or halogen moiety of the allylhalosilane reacts, for example, with hydroxyl groups (e.g. silanol groups) and/or hydrogen groups, particularly hydroxyl groups, on the precipitated silica and the dialkyldihalosilane (e.g. dimethyldichlorosilane) or dialkyldialkoxysilane (e.g. dimethyldiethoxysilane) reacts with, for example, hydroxyl groups (e.g. silanol groups) on the precipitated silica to enhance the hydrophobicity of the treated silica.
The use of a combined treatment of silica with the allylsilane and the dialkyldihalosilane or dialkyldialkoxysilane has been observed in enhanced hydrophobicity of the silica over the use of the allyl silane alone. For example the allyltrichlorosilane used to treat the silica has an area of 168.18 square angstroms when measured by molecular modeling in a hydrocarbon solvent. The dimethylsilane in a form of dimethyldichlorosilane used in the treatment has an area of 138.30 square angstroms when modeled in the same hydrocarbon environment. The smaller molecular size of the dimethyldichlorosilane allows for deeper penetration into the pores of the silica surface and therefore enables reaction with more of the hydroxyl groups present on the silica surface. It is envisioned that this enhanced hydrophobicity results in enhanced dispersion in the elastomer matrix and enhanced interaction between the silica and the elastomer.
Accordingly, a significant aspect of this invention is an interaction of the combined allylsilane with said dialkyldichlorosilane or dialkyldialkoxysilane treated precipitated silica to not only provide a delayed coupling of the treated precipitated silica with diene-based elastomer in the presence of the sulfur curative during the subsequent vulcanization of the rubber composition but to also promote an enhanced hydrophobicity of the allyl hydrocarbon treated precipitated silica to desirably cause a more efficient dispersibility of the treated silica within the rubber composition prior to the aforesaid promoted coupling of the treated silica to the diene-based elastomer to thereby enhance one or more physical properties for the rubber composition.
It is further envisioned that, if desired, because of an absence of available sulfur atoms in the case of where a sulfur containing coupling agent is not used to couple the silica to the elastomer, a beneficially relatively high temperature mixing of the combination of allyl and dialkyl hydrocarbon treated silica with the elastomer(s) can be achieved without an attendant sulfur-promoted viscosity increase of the rubber mixture prior to addition of the sulfur and sulfur cure accelerators to the rubber mixture, a higher temperature mixing of the rubber composition provides an opportunity for more efficient mixing of the rubber composition for a shorter mixing time.
Representative examples of various allylsilanes for preparation of allyl functionalized silica are, for example, allyltriethoxysilane, allyltrimethoxysilane, allyldimethylchlorosilane, allyltrichlorosilane, allylmethyldichlorosilane, diallylchloromethylsilane, diallyldichlorosilane and triallylchlorosilane.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.